Battery fuel gauges are in widespread use to monitor battery conditions in portable electronic devices. Since battery voltage declines as the battery is discharged, terminal voltage is often used as a proxy for the current, or charge capacity, remaining in the battery. The task of making DC measurements of battery capacity has many variations depending on the processing capabilities of the portable electronic devices and the acceptable cost of accurately monitoring battery condition for the particular device.
Sigma-delta modulators are sometimes used for DC measurement. For example, battery current may be sensed by monitoring the voltage through a resistor placed in a circuit for that purpose. The voltage through the sense resistor is modulated by a sigma-delta modulator in order to convert the input analog voltage signal into a digital bitstream. An example of such an arrangement 10 used in the arts is shown in FIG. 1. A sense resistor 12 is connected in the discharge loop of the battery 14. A bitstream from the sigma-delta modulator 16 is fed into an accumulator 18, or decimation filter, the output of which in turn is passed to a counter 20. The output 24 of the counter 20 is read periodically at a regular or irregular interval. The reading is the net cumulative result, giving an indication of the condition of the monitored battery 14.
It is known that a sigma-delta modulator used in DC measurement is subject to spurious noise, also called pattern noise or “idle tone” if its frequency falls into the audio frequency band. This noise is related to the non-white nature of quantization error of the modulator. When pattern noise is present, the accumulated value output by the accumulator may not precisely represent the DC input to the modulator. Since the accumulator-counter arrangement counts how much current has passed through the sense resistor, the frequency of the pattern noise relative to the accumulation time may have a significant impact on the accuracy of the battery condition measurement. Using the current state of the art, if the accumulation time happens to match the period of the pattern noise, a better result is obtained than if the accumulation time and pattern noise cycle are out of synch.
It is known in the arts to estimate the upper and lower bounds of the digital error present in the digital representation of an analog signal. This technique is sometimes used in an effort to reduce error in DC measurements. Such a technique leaves much to be desired, however in terms of measurement accuracy. It would be useful and desirable in the arts to increase the accuracy of DC current monitoring for battery fuel gauges. It would be particularly advantageous if methods, algorithms, and systems were made available for systematically reducing error based on the characteristics of the input signal to be measured.